Physics gives the answer in a flash

As wind turbine technology advances and goes mainstream, it is critical that support technology such as lightning protection advance along with it. While system operators lived with blade vulnerability to strikes ten years ago, now they are less prepared to. Blades of 30 m and longer are common, and the cost of replacing them can exceed $100 000 (80 000 €) each.

Data collected from one winter season in Japan alone reveal losses of unacceptable proportions. In one season, at least 55 machines, at a wind farm in Honshu, had blades destroyed by lightning. The estimated one-year loss for those machines exceeded $5 500 000 (4 500 000 €), not including lost production, cleanup, and other secondary costs.

In at least one recorded case, a turbine blade was destroyed completely by lightning. Because the two remaining blades were still rotating, the resulting unbalance twisted the tower, resulting in a collapse of the monopole and a total loss of the wind turbine system, costing well over a million dollars (shown in photo above).

In another case, when lightning struck a wind turbine directly and started a fire, probably caused by a strong electromagnetic pulse common to positive strikes in that area, the entire wind turbine was destroyed, and the mounting monopole's strength may also be in question, a further significant loss.

Without adequate lightning protection, the same loss of wind turbine equipment, generating capacity, staff resources and probable rise of insurance premiums could occur during the next storm season.

But wind turbines are one of the most challenging machines to protect from lightning because their most sensitive part is not only the most likely target but also the least likely to survive a strike. Since wind turbine blades are not electrical conductors, suppliers try to correct this by inserting conductive wire inside them to dissipate stray electrical energy. But blade failure occurs when the energy dissipated by inserted wire is not enough to handle lightning strikes or related phenomena.

Trying to build a better "lightning trap," lightning rod manufacturers developed a family of strike collectors called early streamer emitters (ESE's), based on the premise that the faster a streamer is generated, the greater the chance of collecting the incoming lightning leader. Unfortunately, ESE's fail to compete with the streamers emitted by turbine blades.

CIGRE states that it cannot support the use of any ESE's because "the theoretical basis for the early streamer emission technology appears technically incorrect." An ESE study authorised by the USA NFPA Lightning Committee (the Bryan Report) concluded that ESE's are no better than lightning rods. Further tests conducted by the New Mexico Institute of Technology in their mountain top lightning lab show that ESE's, in fact, are less effective than a common lightning rod.

However, there are proven lightning protection alternatives. One example is the Ion Plasma Generator (IPG) (pictured top right) developed by USA-based LEC, Lightning Eliminators & Consultants, Inc (www.lightningeliminators.com).

In a typical electrical storm, clouds emit a negatively-charged downward "leader" searching for an attractive, positively-charged, upward streamer or "counter leader" from the ground, to trigger a lightning strike and balance the electrical charge. Since lightning termination requires a rising conductive path from earth to close the circuit, an effective strike collector must generate the most "attractive" path in a competitive situation. Analytical, test, and operational data prove the IPG successfully competes with any form of streamer generated elsewhere.

In contrast with the narrow, weak, upward streamers generated from single-point lightning rods and ESE's, the IPG collector creates a corona plasma, or dense massive flow of attractive ions, from many points spaced and engineered appropriately for collection.

The IPG directs the corona plasma upward to capture direct lightning strikes, influencing the lightning leader path in the last few microseconds of its trip to earth. The dense plasma presents a far more attractive force than wind turbine blades or any other form of streamer.

In competition with a turbine blade, the IPG provides a successful counter leader to distances over 20 times that of the best competition. The IPG is the only strike collector proven to be effective to at least 100 m in a competitive environment, and the only one firmly based on a rigorous analysis of the physics of the problem. Compared with a single lightning rod of the same height, the IPG provides a collection zone several times larger.

Two notable successes involve wind turbines with a height of 72.5 m (238 ft) located in the most lightning vulnerable areas of Japan. There, peak lightning currents have been recorded of up to 200 000 A, and di/dt may have reached 500 000 amperes per microsecond. LEC installed IPG lightning protection systems on two wind turbine sites within a group of 15 in the area. At the time of writing all 13 unprotected wind turbine groups had been struck by lightning, with at least one turbine blade destroyed in each group. To date, the two IPG-protected sites continue to function with no lightning-related losses.

A monopole mounted IPG can cost less than replacing a single 30 m turbine blade, and the guyed tower kit costs half that of the monopole kit. The kits are guaranteed to collect lightning strikes in preference to wind turbine blades, even if the IPG is below blade height but within the protection zone. Both kits provide ultra fast acting surge protection and low impedance grounding protection.